Gear shifting on target speed reduction in vehicle speed control system

ABSTRACT

A vehicle speed control system for a vehicle with an engine and an automatic transmission is comprised of a coast switch for decreasing a set vehicle speed and a controller connected with the coast switch. The controller controls a vehicle speed at the set vehicle speed by controlling the engine and the automatic transmission, and maintains a gear ratio of the automatic transmission at the gear ratio set at the moment before decreasing the set vehicle speed when the coast switch is being operated to decrease the set vehicle speed. Therefore, even if the throttle opening is opened to accelerate the vehicle, the engine rotation speed is never radically increased under such a transmission condition. This prevents the engine from generating noises excessively.

TECHNICAL FIELD

[0001] The present invention relates to a vehicle speed control systemfor controlling a vehicle speed, and more particularly to a vehiclespeed control system which controls a vehicle so as to automaticallycruise the vehicle at a set vehicle speed.

BACKGROUND ART

[0002] A Japanese Patent Provisional Publication No. (Heisei) 1-60437discloses a vehicle speed control system which is arranged todecelerates a vehicle by a shift down control of a transmission inaddition to a throttle control of an engine when a coast switch is forlowering a set speed is switched on.

DISCLOSURE OF INVENTION

[0003] However, when the coast switch is continuously and excessivelyswitched on such that the set speed is excessively lowered than an aimedset speed, it is necessary to increase the lowered set speed to theaimed set speed by switching on an accelerate switch.

[0004] For example, when it was desired to lower the set speed from 80km/h to 60 km/h but when the set speed was excessively lowered to 50km/h by pressing the coast switch six times (80 km/h-6×5 km/h), it isnecessary to press the accelerate switch twice to return the set speedto 60 km/h. Since the shift-down transmission control is started inreply to the lowering operation of the set speed, the transmissionconnected to the conventional vehicle speed control system has alreadyexecuted a shift down operation at the moment when the set speed isincreased.

[0005] Accordingly, in such a situation, the vehicle speed controlsystem outputs an acceleration command to the controlled system. As aresult, the vehicle in the shift-down condition is accelerated byincreasing the throttle opening. This operation will excessivelyincrease the engine rotation speed and excessively generate noises.

[0006] It is therefore an object of the present invention to provide animproved vehicle speed control system which solves the above-mentionedproblem.

[0007] A vehicle speed control system according to the present inventionis for a vehicle equipped with an engine and an automatic transmission,and comprises a coast switch for decreasing a set vehicle speed and acontroller connected with the coast switch. The controller is arrangedto control a vehicle speed at the set vehicle speed by controlling athrottle of the engine and the automatic transmission, and to maintain agear ratio of the automatic transmission at the gear ratio set at themoment before decreasing the set vehicle speed when the coast switch isbeing operated to decrease the set vehicle speed.

BRIEF DESCRIPTION OF DRAWINGS

[0008]FIG. 1 is a block diagram showing a structure of a vehicle speedcontrol system according to the present invention.

[0009]FIG. 2 is a block diagram showing a structure of alateral-acceleration vehicle-speed correction-quantity calculating block580.

[0010]FIG. 3 is a graph showing a relationship between a vehicle speedV_(A)(t) and a cutoff frequency fc of a low pass filter.

[0011]FIG. 4 is a graph showing a relationship between a correctioncoefficient CC for calculating a vehicle speed correction quantityV_(SUB)(t) and a value Y_(G)(t) of the lateral acceleration.

[0012]FIG. 5 is a graph showing a relationship between a naturalfrequency ω_(nSTR) and the vehicle speed.

[0013]FIG. 6 is a graph showing a relationship between an absolute valueof a deviation between vehicle speed V_(A)(t) and a maximum valueV_(SMAX) of a command vehicle speed, and a command vehicle speedvariation ΔV_(COM)(t).

[0014]FIG. 7 is a block diagram showing a structure of a command drivetorque calculating block 530.

[0015]FIG. 8 is a map showing an engine nonlinear stationarycharacteristic.

[0016]FIG. 9 is a map showing an estimated throttle opening.

[0017]FIG. 10 is a map showing a shift map of a CVT.

[0018]FIG. 11 is a map showing an engine performance.

[0019]FIG. 12 is a block diagram showing another structure of commanddrive torque calculating block 530.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Referring to FIGS. 1 to 12, there is shown a vehicle speedcontrol system according to an embodiment of the present invention.

[0021]FIG. 1 shows a block diagram showing a construction of the vehiclespeed-control system according to the embodiment of the presentinvention. With reference to FIGS. 1 to 12, the construction andoperation of the vehicle speed control system according to the presentinvention will be discussed hereinafter.

[0022] The vehicle speed control system according to the presentinvention is equipped on a vehicle and is put in a standby mode in amanner that a vehicle occupant manually switches on a system switch (notshown) of the speed control system. Under this standby mode, when a setswitch 20 is switched on, the speed control system starts operations.

[0023] The vehicle speed control system comprises a vehicle speedcontrol block 500 which is constituted by a microcomputer and peripheraldevices. Blocks in vehicle speed control block 500 represent operationsexecuted by this microcomputer. Vehicle speed control block 500 receivessignals from a steer angle sensor 100, a vehicle speed sensor 10, theset switch 20, a coast switch 30, an accelerate (ACC) switch 40, anengine speed sensor 80, an accelerator pedal sensor 90 and acontinuously variable transmission (CVT) 70. According to the signalsreceived, vehicle speed control block 500 calculates various commandvalues and outputs these command values to CVT 70, a brake actuator 50and a throttle actuator 60 of the vehicle, respectively, to control anactual vehicle speed at a target vehicle speed.

[0024] A command vehicle speed determining block 510 of vehicle speedcontrol block 500 calculates a command vehicle speed V_(COM)(t) by eachcontrol cycle, such as by 10 ms. A suffix (t) denotes that the valuewith the suffix (t) is a valve at the time t and is varied in timeseries (time elapse). In some graphs, such suffix (t) is facilitated.

[0025] A command vehicle speed maximum value setting block 520 sets avehicle speed V_(A)(t) as a command vehicle speed maximum value V_(SMAX)(target speed) at time when set switch 30 is switched on. Vehicle speedV_(A)(t) is an actual vehicle speed which is detected from a rotationspeed of a tire rotation speed by means of a vehicle speed sensor 10.

[0026] After command vehicle speed maximum value V_(SMAX) is set by theoperation of set switch 20, command vehicle speed setting block 520decreases command vehicle speed maximum value V_(SMAX) by 5 km/h inreply to one push of coast switch 30. That is, when coast switch 30 ispushed a number n of times (n times), command vehicle speed V_(SMAX) isdecreased by n×5 km/h. Further, when coast switch 30 has been pushed fora time period T (sec.), command vehicle speed V_(SMAX) is decreased by avalue T/1(sec.)×5 km/h.

[0027] Similarly, after command vehicle speed maximum value V_(SMAX) isset by the operation of set switch 20, command vehicle speed settingblock 520 increases command vehicle speed maximum value V_(SMAX) by 5km/h in reply to one push of ACC switch 40. That is, when ACC switch 40is pushed a number n of times (n times), command vehicle speed maximumvalue V_(SMAX) is increased by n×5 km/h. Further, ACC switch 40 has beenpushed for a time period T (sec.), command vehicle speed maximum valueV_(SMAX) is increased by a value T/1(sec.)×5 (km/h).

[0028] A lateral acceleration (lateral G) vehicle-speedcorrection-quantity calculating block 580 receives a steer angle θ(t)from steer angle-sensor 100 and vehicle speed V_(A)(t) from vehiclespeed sensor 10, and calculates a vehicle speed correction quantityV_(SUB)(t) which is employed to correct the command vehicle speedV_(COM)(t) according to a lateral acceleration (hereinafter, it called alateral-G). More specifically, lateral-G vehicle-speedcorrection-quantity calculating section 580 comprises a steer anglesignal low-pass filter (hereinafter, it called a steer angle signal LPFblock) 581, a lateral-G calculating block 582 and a vehicle speedcorrection quantity calculation map 583, as shown in FIG. 2.

[0029] Steer angle signal LPF block 581 receives vehicle speed V_(A)(t)and steer angle θ(t) and calculates a steer angle LPF value θ_(LPF)(t).Steer angle LPF value θ_(LPF)(t) is represented by the followingequation (1).

θ_(LPF)(t)=θ(t)/(TSTR·s+1)  (1)

[0030] In this equation (1), s is a differential operator, and TSTR is atime constant of the low-pass filter (LPF) and is represented byTSTR=1/(2π·fc). Further, fc is a cutoff frequency of LPF and isdetermined according to vehicle speed V_(A)(t) as shown by a map showinga relationship between cutoff frequency fc and vehicle speed V_(A)(t) inFIG. 3. As is clear from the map of FIG. 3, cutoff frequency fc becomessmaller as the vehicle speed becomes higher.

[0031] For example, a cutoff frequency at the vehicle speed 100 km/h issmaller than that at the vehicle speed 50 km/h.

[0032] Lateral-G calculating block 582 receives steer angle LPF valueθ_(LPF)(t) and vehicle speed V_(A)(t) and calculates the lateral-GY_(G)(t) from the following equation (2).

Y _(G)(t)={V _(A)(t)²·θ_(LPF)(t)}/{N·W·[1+A·V _(A)(t)²]}  (2)

[0033] In this equation (2), W is a wheelbase dimension of the vehicle,N is a steering gear ratio, and A is a stability factor. The equation(2) is employed in case that the lateral G of the vehicle is obtainedfrom the steer angle.

[0034] When the lateral G is obtained by using a yaw-rate sensor andprocessing the yaw rate ψ(t) by means of a low-pass filter (LPF), thelateral-G Y_(G)(t) is obtained from the following equations (3) and (4).

Y _(G)(t)=V _(A)(t)·ψ_(LPF)  (3)

ψ_(LPF)=ψ(t)/(T _(YAW) ·s+1)  (4)

[0035] In the equation (4), T_(YAW)is a time constant of the low-passfilter. The time constant T_(YAW) increases as vehicle speed V_(A)(t)increases.

[0036] Vehicle speed correction calculation map 583 calculates a vehiclespeed correction quantity V_(SUB)(t) which is employed to correctcommand vehicle speed V_(COM)(t) according to lateral-G Y_(G)(t).Vehicle speed correction quantity V_(SUB)(t) is calculated bymultiplying a correction coefficient CC determined from the lateral Gand a predetermined variation limit of command vehicle speed V_(COM)(t).In this embodiment, the predetermined variation limit of command vehiclespeed V_(COM)(t) is set at 0.021 (km/h/10 ms)=0.06 G. The predeterminedvariation limit of the command vehicle speed is equal to the maximumvalue of a variation (corresponding to acceleration/deceleration)ΔV_(COM)(t) of the command vehicle speed shown in FIG. 6.

V _(SUB)(t)=CC×0.021(km/h/10 ms)  (5)

[0037] As mentioned later, the vehicle speed correction quantityV_(SUB)(t) is added as a subtraction term in the calculation process ofthe command vehicle speed V_(COM)(t) which is employed to control thevehicle speed. Accordingly, command vehicle speed V_(COM)(t) is limitedto a smaller value as vehicle correction quantity V_(SUB)(t) becomeslarger.

[0038] Correction coefficient CC becomes larger as lateral-G Y_(G)becomes larger, as shown in FIG. 4. The reason thereof is that thechange of command vehicle speed V_(COM)(t) is limited more as thelateral-G becomes larger. However, when the lateral-G is smaller than orequal to 0.1G as shown in FIG. 4, correction coefficient CC is set atzero since it is decided that it is not necessary to correct commandvehicle speed V_(COM)(t). Further, when the lateral-G is greater than orequal to 0.3G, correction coefficient CC is set at a predeterminedconstant value. That is, the lateral-G never becomes greater than orequal to 0.3G as far as the vehicle is operated under a usual drivingcondition. Therefore, in order to prevent the correction coefficient CCfrom being set at an excessively large value when the detection value ofthe lateral-G erroneously becomes large, the correction coefficient CCis set at such a constant value, such as at 2.

[0039] When a driver requests to increase the target vehicle speed byoperating accelerate switch 40, that is, when acceleration of thevehicle is requested, the command vehicle speed V_(COM)(t) is calculatedby adding present vehicle speed V_(A)(t) and command vehicle speedvariation ΔV_(COM)(t) and by subtracting vehicle speed correctionquantity V_(SUB)(t) from the sum of present vehicle speed V_(A)(t) andcommand vehicle speed variation ΔV_(COM)(t).

[0040] Therefore, when command vehicle speed variation ΔV_(COM)(t) isgreater than vehicle speed correction quantity V_(SUB)(t), the vehicleis accelerated. When command vehicle speed variation ΔV_(COM)(t) issmaller than vehicle speed correction quantity V_(SUB)(t), the vehicleis decelerated. Vehicle speed correction quantity V_(SUB)(t) is obtainedby multiplying the limit value of the command vehicle speed variation (amaximum value of the command vehicle speed variation) with correctioncoefficient CC shown in FIG. 4. Therefore, when the limit value of thecommand vehicle speed variation is equal to the command vehicle speedvariation and when correction coefficient CC is 1, the amount foracceleration becomes equal to the amount for deceleration. In case ofFIG. 4, when Y_(G)(t)=0.2, the amount for acceleration becomes equal tothe amount for deceleration. Accordingly, the present vehicle speed ismaintained when the correction coefficient CC is 1. In this example,when the lateral-G Y_(G)(t) is smaller than 0.2, the vehicle isaccelerated. When the lateral-G Y_(G)(t) is larger than 0.2, the vehicleis decelerated.

[0041] When the driver requests to lower the target vehicle speed byoperating coast switch 30, that is, when the deceleration of the vehicleis requested, the command-vehicle speed V_(COM)(t) is calculated bysubtracting command vehicle speed variation ΔV_(COM)(t) and vehiclespeed correction quantity V_(SUB)(t) from present vehicle speedV_(A)(t). Therefore, in this case, the vehicle is always decelerated.The degree of the deceleration becomes larger as vehicle speedcorrection quantity V_(SUB)(t) becomes larger. That is, vehicle speedcorrection quantity V_(SUB)(t) increases according to the increase ofthe lateral-G Y_(G)(t). The above-mentioned value 0.021(km/h/10 ms) hasbeen defined on the assumption that the vehicle is traveling on ahighway.

[0042] As mentioned above, vehicle speed correction quantity V_(SUB)(t)is obtained from the multiple between the correction coefficient CCaccording to the lateral acceleration and the limit value of the commandvehicle speed variation V_(COM)(t). Accordingly, the subtract term(vehicle speed correction quantity) increases according to the increaseof the lateral acceleration so that the vehicle speed is controlled soas to suppress the lateral-G. However, as mentioned in the explanationof steer angle signal LPF block 581, the cutoff frequency fc is loweredas the vehicle speed becomes larger. Therefore the time constant TSTR ofthe LPF is increased, and the steer angle LPF θ_(LPF)(t) is decreased.Accordingly, the lateral acceleration estimated at the lateral-Gcalculating block 581 is also decreased. As a result, the vehicle speedcorrection quantity V_(SUB)(t), which is obtained from the vehicle speedcorrection quantity calculation map 583, is decreased.

[0043] Consequently, the steer angle becomes ineffective as to thecorrection of the command vehicle speed. In other words, the correctiontoward the decrease of the acceleration becomes smaller due to thedecrease of vehicle speed correction quantity V_(SUB)(t).

[0044] More specifically, the characteristic of the natural frequencyω_(nSTR) relative to the steer angle is represented by the followingequation (6).

ω_(nSTR)=(2W/V _(A)){square root}{square root over ([Kf·Kr·(1+A·V _(A)²)/m _(V)·I])}  (6)

[0045] In this equation (6), Kf is a cornering power of one front tire,Kr is a cornering power of one rear tire, W is a wheelbase dimension,m_(v) is a vehicle weight, A is a stability factor, and I is a vehicleyaw inertia moment.

[0046] The characteristic of the natural frequency ω_(nSTR) performssuch that the natural frequency ω_(nSTR) becomes smaller and the vehicleresponsibility relative to the steer angle degrades as the vehicle speedincreases, and that the natural frequency ω_(nSTR) becomes greater andthe vehicle responsibility relative to the steer angle is improved asthe vehicle speed decreases. That is, the lateral-G tends to begenerated according to a steering operation as the vehicle speed becomeslower, and the lateral-G due to the steering operation tends to besuppressed as the vehicle speed becomes higher. Therefore, the vehiclespeed control system according to the present invention is arranged tolower the responsibility by decreasing the cutoff frequency fc accordingto the increase of the vehicle speed so that the command vehicle speedtends not to be affected by the correction due to the steer angle as thevehicle speed becomes higher.

[0047] A command vehicle speed variation determining block 590 receivesvehicle speed V_(A)(t) and command vehicle speed maximum value V_(SMAX)and calculates the command vehicle speed variation ΔV_(COM)(t) from themap shown in FIG. 6 on the basis of an absolute value |V_(A)−V_(SMAX)|of a deviation between the vehicle speed V_(A)(t) and the commandvehicle speed maximum value V_(SMAX).

[0048] The map for determining command vehicle speed variationΔV_(COM)(t) is arranged as shown in FIG. 6. More specifically, whenabsolute value |V_(A)−V_(SMAX)| of the deviation is within a range B inFIG. 6, the vehicle is quickly accelerated or decelerated by increasingcommand vehicle speed variation ΔV_(COM)(t) as the absolute value of thedeviation between vehicle speed V_(A)(t) and command vehicle speedmaximum value V_(SMAX) is increased within a range where command vehiclespeed variation ΔV_(COM)(t) is smaller than acceleration limit α fordeciding the stop of the vehicle speed control. Further, when theabsolute value of the deviation is small within the range B in FIG. 6,command vehicle speed variation ΔV_(COM)(t) is decreased as the absolutevalue of the deviation decreases within a range where the driver canfeel an acceleration of the vehicle and the command vehicle speedvariation ΔV_(COM)(t) does not overshoot maximum value V_(SMAX) of thecommand vehicle speed. When the absolute value of the deviation is largeand within a range A in FIG. 6, command vehicle speed variationΔV_(COM)(t) is set at a constant value which is smaller thanacceleration limit a, such as at 0.06G. When the absolute value of thedeviation is small and within a range C in FIG. 6, command vehicle speedvariation ΔV_(COM)(t) is set at a constant value, such as at 0.03G.

[0049] Command vehicle speed variation determining block 590 monitorsvehicle speed correction quantity V_(SUB)(t) outputted from lateral-Gvehicle speed correction quantity calculating block 580, and decidesthat a traveling on a curved road is terminated when vehicle speedcorrection quantity V_(SUB)(t) is returned to zero after vehicle speedcorrection quantity V_(SUB)(t) took a value except for zero from zero.Further, command vehicle speed variation determining block 590 detectswhether vehicle speed V_(A)(t) becomes equal to maximum value V_(SMAX)of the command vehicle speed.

[0050] When it is decided that the traveling on a curved road isterminated, the command vehicle speed variation ΔV_(COM)(t) iscalculated from vehicle speed V_(A)(t) at the moment when it is decidedthat the traveling on a curved road is terminated, instead ofdetermining the command vehicle speed variation ΔV_(COM)(t) by using themap of FIG. 6 on the basis of the absolute value of a deviation betweenvehicle speed V_(A)(t) and maximum value V_(SMAX) of the command vehiclespeed. The characteristic employed for calculating the command vehiclespeed variation ΔV_(COM)(t) under the curve-traveling terminatedcondition performs a tendency which is similar to that of FIG. 6. Morespecifically, in this characteristic employed in this curve terminatedcondition, a horizontal axis denotes vehicle speed V_(A)(t) instead ofabsolute value |V_(A)(t)−V_(SMAX)|. Accordingly, command vehicle speedvariation ΔV_(COM)(t) becomes small as vehicle speed V_(A)(t) becomessmall. This processing is terminated when vehicle speed V_(A)(t) becomesequal to maximum value V_(SMAX) of the command vehicle speed.

[0051] Instead of the above determination method of command vehiclespeed variation ΔV_(COM)(t) at the termination of the curved roadtraveling, when vehicle speed correction quantity V_(SUB)(t) takes avalue except for zero, it is decided that the curved road traveling isstarted. Under this situation, vehicle speed V_(A)(t1) at a moment t1 ofstarting the curved road traveling may be previously stored, and commandvehicle speed variation ΔV_(COM)(t) may be determined from a magnitudeof a difference ΔV_(A) between vehicle speed V_(A)(t1) at the moment t1of the start of the curved road traveling and vehicle speed V_(A)(t2) atthe moment t2 of the termination of the curved road traveling. Thecharacteristic employed for calculating the command vehicle speedvariation ΔV_(COM)(t) under this condition performs a tendency which isopposite to that of FIG. 6. More specifically, in this characteristiccurve, there is employed a map in which a horizontal axis denotesvehicle speed V_(A)(t) instead of |V_(A)(t)−V_(SMAX)|. Accordingly,command vehicle speed variation ΔV_(COM)(t) becomes smaller as vehiclespeed V_(A)(t) becomes larger. This processing is terminated whenvehicle speed V_(A)(t) becomes equal to maximum value V_(SMAX) of thecommand vehicle speed.

[0052] That is, when the vehicle travels on a curved road, the commandvehicle speed is corrected so that the lateral-G is suppressed within apredetermined range. Therefore, the vehicle speed is lowered in thissituation generally. After the traveling on a curved road is terminatedand the vehicle speed is decreased, the command vehicle speed variationΔV_(COM)(t) is varied according to vehicle speed V_(A)(t) at the momentof termination of the curved road traveling or according to themagnitude of the difference ΔV_(A) between vehicle speed V_(A)(t1) atthe moment t1 of staring of the curved road traveling and vehicle speedV_(A)(t2) at the moment t2 of the termination of the curved roadtraveling.

[0053] Further, when the vehicle speed during the curved road travelingis small or when vehicle speed difference ΔV_(A) is small, commandvehicle speed variation ΔV_(COM)(t) is set small and therefore theacceleration for the vehicle speed control due to the command vehiclespeed is decreased. This operation functions to preventing a largeacceleration from being generated by each curve when the vehicle travelson a winding road having continuous curves such as a S-shape curvedroad. Similarly, when the vehicle speed is high at the moment of thetermination of the curved road traveling, or when vehicle speeddifference ΔV_(A) is small, it is decided that the traveling curve issingle and command vehicle speed variation ΔV_(COM)(t) is set at a largevalue. Accordingly, the vehicle is accelerated just after the travelingof a single curved road is terminated, and therefore the driver of thevehicle becomes free from a strange feeling due to the slow-down of theacceleration.

[0054] Command vehicle speed determining block 510 receives vehiclespeed V_(A)(t), vehicle speed correction quantity V_(SUB)(t), commandvehicle speed variation ΔV_(COM)(t) and maximum value V_(SMAX) of thecommand vehicle speed and calculates command vehicle speed V_(COM)(t) asfollows.

[0055] (a) When maximum value V_(SMAX) of the command vehicle speed isgreater than vehicle speed V_(A)(t), that is, when the driver requestsaccelerating the vehicle by operating accelerate switch 40 (or a resumeswitch), command vehicle speed V_(COM)(t) is calculated from thefollowing equation (7).

V _(COM)(t)=min[V _(SMAX) , V _(A)(t)+ΔV _(COM)(t)−V _(SUB)(t)]  (7)

[0056] That is, smaller one of maximum value V_(SMAX) and the value[V_(A)(t)+ΔV_(COM)(t)−V_(SUB)(t)] is selected as command vehicle speedV_(COM)(t).

[0057] (b) When V_(SMAX)=V_(A)(t), that is, when the vehicle travels ata constant speed, command vehicle speed V_(COM)(t) is calculated fromthe following equation (8).

V _(COM)(t)=V _(SMAX) −V _(SUB)(t)  (8)

[0058] That is, command vehicle speed V_(COM)(t) is obtained bysubtracting vehicle speed correction quantity V_(SUB)(t) from maximumvalue V_(SMAX) of the command vehicle speed.

[0059] (c) When maximum value V_(SMAX) of the command vehicle speed issmaller than vehicle speed V_(A)(t), that is, when the driver requeststo decelerate the vehicle by operating coast switch 30, command vehiclespeed V_(COM)(t) is calculated from the following equation (9).

V _(COM)(t)=max[V _(SMAX) , V _(A)(t)−ΔV _(COM)(t)−V _(SUB)(t)]  (9)

[0060] That is, larger one of maximum value V_(SMAX) and the value[V_(A)(t)−ΔV_(COM)(t)−V_(SUB)(t)] is selected as command vehicle speedV_(COM)(t).

[0061] Command vehicle speed V_(COM)(t) is determined from theabove-mentioned manner, and the vehicle speed control system controlsvehicle speed V_(A)(t) according to the determined command vehicle speedV_(COM)(t).

[0062] A command drive torque calculating block 530 of vehicle speedcontrol block 500 in FIG. 1 receives command vehicle speed V_(COM)(t)and vehicle speed V_(A)(t) and calculates a command drive torqued_(FC)(t). FIG. 7 shows a construction of command drive torquecalculating block 530.

[0063] When the input is command vehicle speed V_(COM)(t) and the outputis vehicle speed V_(A)(t), a transfer characteristic (function) G_(V)(s)thereof is represented by the following equation (10).

G _(V)(s)=1/(T _(V) ·s+1)·e ^((−Lv·s))  (10)

[0064] In this equation (10), T_(V) is a first-order lag time constant,and L_(V) is a dead time due to a delay of a power train system.

[0065] By modeling a vehicle model of a controlled system in a manner oftreating command drive torque d_(FC)(t) as a control input (manipulatedvalue) and vehicle speed V_(A)(t) as a controlled value, the behavior ofa vehicle power train is represented by a simplified linear model shownby the following equation (11).

V _(A)(t)=1/(m _(V) ·Rt·s)·e ^((−Lv·s)) ·d _(FC)(t)  (1)

[0066] In this equation (11), Rt is an effective radius of a tire, andm_(V) is a vehicle mass (weight).

[0067] The vehicle model, which employs command drive torque d_(FC)(t)as an input and vehicle speed V_(A)(t) as an output, performs anintegral characteristic since the equation (11) of the vehicle model isof a 1/s type.

[0068] Although the controlled system (vehicle) performs a non-linearcharacteristic which includes a dead time L_(V) due to the delay of thepower train system and varies the dead time L_(V) according to theemployed actuators and engine, the vehicle model, which employs thecommand drive torque d_(FC)(t) as an input and vehicle speed V_(A)(t) asan output, can be represented by the equation (11) by means of theapproximate zeroing method employing a disturbance estimator.

[0069] By corresponding the response characteristic of the controlledsystem of employing the command drive torque d_(FC)(t) as an input andvehicle speed V_(A)(t) as an output to a characteristic of the transferfunction G_(V)(s) having a predetermined first-order lag T_(V) and thedead time L_(V), the following relationship is obtained by using C₁(s),C₂(s) and C₃(s) shown in FIG. 7.

C ₁(s)=e ^((−Lv·s))/(T _(H) ·s+1)  (12)

C ₂(s)=(m _(V) ·Rt·s)/(T _(H) ·s+1)  (13)

d _(V)(t)=C ₂(s)·V_(A)(t)−C₁(s)·d _(FC)(t)  (14)

[0070] In these equations (12), (13) and (14), C₁(s) and C₂(s) aredisturbance estimators for the approximate zeroing method and perform asa compensator for suppressing the influence due to the disturbance andthe modeling.

[0071] When a norm model G_(V)(s) is treated as a first-order low-passfilter having a time constant T_(V) upon neglecting the dead time of thecontrolled system, the model matching compensator C₃(s) takes a constantas follows.

C ₃(t)=m_(V) ·Rt/T _(V)  (15)

[0072] From these compensators C₁(s), C₂(s) and C₃(s), the command drivetorque d_(FC)(t) is calculated from the following equation (16)

d _(FC)(t)=C ₃(s)·{V _(COM)(t)−V _(A)(t)}−{C ₂(s)·V _(A)(t)−C ₁(s)·d_(FC)(t)}  (16)

[0073] A drive torque of the vehicle is controlled on the basis ofcommand drive torque d_(FC)(t). More specifically, the command throttleopening is calculated so as to bring actual drive torque d_(FA)(t)closer to command drive torque d_(FC)(t) by using a map indicative of anengine non-linear stationary characteristic. This map is shown in FIG.8, the relationship represented by this map has been previously measuredand stored. Further, when the required toque is negative and is notensured by the negative drive torque of the engine, the vehicle controlsystem operates the transmission and the brake system to ensure therequired negative torque. Thus, by controlling the throttle opening, thetransmission and the brake system, it becomes possible to modify theengine non-linear stationary characteristic into a linearizedcharacteristic.

[0074] Since CVT 70 employed in this embodiment according to the presentinvention is provided with a torque converter with a lockup mechanism,vehicle speed control block 500 receives a lockup signal LUs from acontroller of CVT 70. The lockup signal LUs indicates the lockupcondition of CVT 70. When vehicle speed control block 500 decides thatCVT 70 is put in an un-lockup condition on the basis of the lockupsignal LU_(S), vehicle speed control block 500 increases the timeconstant T_(H) employed to represent the compensators C₁(s) and C₂(s) asshown in FIG. 7. The increase of the time constant T_(H) decreases thevehicle speed control feedback correction quantity, which corresponds toa correction coefficient for keeping a desired response characteristic.Therefore, it becomes possible to adjust the model characteristic to theresponse characteristic of the controlled system under the un-lockupcondition, although the response characteristic of the controlled systemunder the un-lockup condition delays as compared with that of thecontrolled system under the lockup condition. Accordingly, the stabilityof the vehicle speed control system is ensured under both lockupcondition and un-lockup condition.

[0075] Command drive torque calculating block 530 shown in FIG. 7 isconstructed by compensators C₁(s) and C₂(s) for compensating thetransfer characteristic of the controlled system and compensator C₃(s)for achieving a response characteristic previously designed by adesigner.

[0076] Further, command drive torque calculating block 530 may beconstructed by a pre-compensator C_(F)(s) for compensating so as toensure a desired response characteristic determined by the designer, anorm model calculating block C_(R)(s) for calculating the desiredresponse characteristic determined by the designer and a feedbackcompensator C₃ (s)′ for compensating a drift quantity (a differencebetween the target vehicle speed and the actual vehicle speed) withrespect to the response characteristic of the norm model calculatingsection C_(R)(s), as shown in FIG. 12.

[0077] The pre-compensator C_(F)(s) calculates a standard command drivetorque d_(FC1)(t) by using a filter represented by the followingequation (17), in order to achieve the transfer function G_(V)(s) of theactual vehicle speed V_(A)(t) with respect to the command vehicle speedV_(COM)(t).

d _(FC1)(t)=m _(V) ·R _(T) ·s·V _(COM)(t)/(T _(V) ·s+1)  (17)

[0078] Norm model calculating block C_(R)(s) calculates a targetresponse V_(T)(t) of the vehicle speed control system from the transferfunction G_(V)(s) and the command vehicle speed V_(COM)(t) as follows.

V _(T)(t)=G _(V)(s)·V _(COM)(t)  (18)

[0079] Feedback compensator C₃(s)′ calculates a correction quantity ofthe command drive torque so as to cancel a deviation thereby when thedeviation between the target response V_(T)(t) and the actual vehiclespeed V_(A)(t) is caused. That is, the correction quantity d_(V)(t)′ iscalculated from the following equation (19).

d _(V)(t)′=[(K _(P) ′·s+K _(I)′)/s][V _(T)(t)−V _(A)(t)]  (19)

[0080] In this equation (19), K_(P) is a proportion control gain of thefeedback compensator C₃(s)′, K_(I) is an integral control gain of thefeedback compensator C₃(s)′, and the correction quantity d_(V)(t)′ ofthe drive torque corresponds to an estimated disturbance d_(V)(t) inFIG. 7.

[0081] When it is decided that CVT 70 is put in the un-lockup conditionfrom the lockup condition signal LUs, the correction quantity d_(V)(t)′is calculated from the following equation (20).

d _(V)(t)′=[(K _(P) ′·s+K _(I)′)/s][V _(T)(t)−V _(A)(t)]  (20)

[0082] In this equation (20), K_(P)′>K_(P), and K_(I)′>K_(I). Therefore,the feedback gain in the un-lockup condition of CVT 70 is decreased ascompared with that in the-lockup condition of CVT 70. Further, commanddrive torque d_(FC)(t) is calculated from a standard command drivetorque d_(FC)(t) and the correction quantity d_(V)(t)′ as follows.

d _(FC)(t)=d _(FC1)(t)+d_(V)(t)′  (21)

[0083] That is, when CVT 70 is put in the un-lockup condition, thefeedback gain is set at a smaller value as compared with the feedbackgain in the lockup condition. Accordingly, the changing rate of thecorrection quantity of the command drive torque becomes smaller, andtherefore it becomes possible to adapt the response characteristic ofthe controlled system which characteristic delays under the un-lockupcondition of CVT 70 as compared with the characteristic in the lockupcondition. Consequently, the stability of the vehicle speed controlsystem is ensured under both of the lockup condition and the un-lockupcondition.

[0084] Next, the actuator drive system of FIG. 1 will be discussedhereinafter.

[0085] A command gear ratio calculating block 540 of vehicle speedcontrol block 500 in FIG. 1 receives command drive torque d_(FC)(t),vehicle speed V_(A)(t), the output of coast switch 30 and the output ofaccelerator pedal sensor 90. Command gear ratio calculating block 540calculates a command gear ratio DRATIO(t), which is a ratio of an inputrotation speed and an output rotation speed of CVT 70, on the basis ofthe received information and outputs command gear ratio DRATIO(t) to CVT70 as mentioned hereinafter.

[0086] (a) When coast switch 30 is put in an off state, an estimatedthrottle opening TVO_(ESTI) is calculated from the throttle openingestimation map shown in FIG. 9 on the basis of vehicle speed V_(A)(t)and command drive torque d_(FC)(t). Then, a command engine rotationspeed N_(IN-COM) is calculated from the CVT shifting map shown in FIG.10 on the basis of estimated throttle opening TVO_(ESTI) and vehiclespeed V_(A)(t). Further, command gear ratio DRATIO(t) is obtained fromthe following equation (22) on the basis of vehicle speed V_(A)(t) andcommand engine rotation speed N_(IN-COM).

DRATIO(t)=N _(IN-COM)·2π·Rt/[60·V _(A)(t)·Gf]  (22)

[0087] In this equation (22), Gf is a final gear ratio.

[0088] (b) When coast switch 30 is put in an on state, that is, whenmaximum value V_(SMAX) of the command vehicle speed is decreased byswitching on coast switch 30, the previous value DRATIO(t−1) of commandgear ratio is maintained as the present command gear ratio DRATIO(t).Therefore, even when coast switch 30 is continuously switched on,command gear ratio DRATIO(t) is maintained at the value set just beforethe switching on of coast switch 30 until coast switch is switched off.That is, the shift down is prohibited for a period from the switching onof coast switch 30 to the switching off of coast switch 30.

[0089] More specifically, when the set speed of the vehicle speedcontrol system is once decreased by operating coast switch 30 and isthen increased by operating accelerate switch 40, the shift down isprohibited during this period. Therefore, even if the throttle openingis opened to accelerate the vehicle, the engine rotation speed is neverradically increased under such a transmission condition. This preventsthe engine from generating noises excessively.

[0090] An actual gear ratio calculating block 550 of FIG. 1 calculatesan actual gear ratio RATIO(t), which is a ratio of an actual inputrotation speed and an actual output rotation speed of CVT 70, from thefollowing equation on the basis of the engine rotation speed N_(E)(t)and vehicle speed V_(A)(t) which is obtained by detecting an enginespark signal through engine speed sensor 80.

RATIO(t)=N _(E)(t)/[V _(A)(t)·Gf·2π·Rt]  (23)

[0091] A command engine torque calculating block 560 of FIG. 1calculates a command engine torque TE_(COM)(t) from command drive torqued_(FC)(t), actual gear ratio RATIO(t) and the following equation (24).

TE_(COM)(t)=d_(FC)(t)/[Gf·RATIO(t)]  (24)

[0092] A target throttle opening calculating block 570 of FIG. 1calculates a target throttle opening TVO_(COM) from the engineperformance map shown in FIG. 11 on the basis of command engine torqueTE_(COM)(t) and engine rotation speed N_(E)(t), and outputs thecalculated target throttle opening TVO_(COM) to throttle actuator 60.

[0093] A command brake pressure calculating block 630 of FIG. 1calculates an engine brake torque TE_(COM)′ during a throttle fullclosed condition from the engine performance map shown in FIG. 11 on thebasis of engine rotation speed N_(E)(t). Further, command brake pressurecalculating block 630 calculates a command brake pressure REF_(PBRK)(t)from the throttle full-close engine brake torque TE_(COM)′, commandengine torque TE_(COM)(t) and the following equation (25).

REF _(PBRK)(t)=(TE _(COM) −TE _(COM))·Gm·Gf/{4·(2·AB·RB·μB)}  (25)

[0094] In this equation (25), Gm is a gear ratio of CVT 70, AB is awheel cylinder force (cylinder pressure×area), RB is an effective radiusof a disc rotor, and μB is a pad friction coefficient.

[0095] Next, the suspending process of the vehicle speed control will bediscussed hereinafter.

[0096] A vehicle speed control suspension deciding block 620 of FIG. 1receives an accelerator control input APO detected by accelerator pedalsensor 90 and compares accelerator control input APO with apredetermined value. The predetermined value is an accelerator controlinput APO₁ corresponding to a target throttle opening TVO_(COM) inputtedfrom a target throttle opening calculating block 570, that is a throttleopening corresponding to the vehicle speed automatically controlled atthis moment. When accelerator control input APO is greater than apredetermined value, that is, when a throttle opening becomes greaterthan a throttle opening controlled by throttle actuator 60 due to theaccelerator pedal depressing operation of the drive, vehicle speedcontrol suspension deciding block 620 outputs a vehicle speed controlsuspending signal.

[0097] Command drive torque calculating block 530 and target throttleopening calculating block 570 initialize the calculations, respectivelyin reply to the vehicle speed control suspending signal, and thetransmission controller of CVT 70 switches the shift-map from a constantspeed traveling shift-map to a normal traveling shift map. That is, thevehicle speed control system according to the present invention suspendsthe constant speed traveling, and starts the normal traveling accordingto the accelerator pedal operation of the driver.

[0098] The transmission controller of CVT 70 has stored the normaltraveling shift map and the constant speed traveling shift map, and whenthe vehicle speed control system according to the present inventiondecides to suspend the constant vehicle speed control, the vehicle speedcontrol system commands the transmission controller of CVT 70 to switchthe shift map from the constant speed traveling shift map to the normaltraveling shift map. The normal traveling shift map has a highresponsibility characteristic so that the shift down is quickly executedduring the acceleration. The constant speed traveling shift map has amild characteristic which impresses a smooth and mild feeling to adriver when the shift map is switched from the constant speed travelingmode to the normal traveling mode.

[0099] Vehicle speed control suspension deciding block 620 stopsoutputting the vehicle speed control suspending signal when theaccelerator control input APO returns to a value smaller than thepredetermined value. Further, when the accelerator control input APO issmaller than the predetermined value and when vehicle speed V_(A)(t) isgreater than the maximum value V_(SMAX) of the command vehicle speed,vehicle speed control suspension deciding block 620 outputs thedeceleration command to the command drive torque calculating block 530.

[0100] When the output of the vehicle speed control suspending signal isstopped and when the deceleration command is outputted, command drivetorque calculating block 530 basically executes the deceleration controlaccording to the throttle opening calculated at target throttle openingcalculating block 570 so as to achieve command drive torque d_(FC)(t).However, when command drive torque d_(FC)(t) cannot be achieved only byfully closing the throttle, the transmission control is further employedin addition to the throttle control. More specifically, in such a largedeceleration force required condition, command gear ratio calculatingblock 540 outputs the command gear ratio DRATIO (shift down command)regardless the road gradient, such as traveling on a down slope or aflat road. CVT 70 executes the shift down control according to thecommand gear ratio DRATIO to supply the shortage of the deceleratingforce.

[0101] In addition to the above arrangement of employing the shift downcontrol of CVT 70 based on the magnitude of the deceleration in therestarting operation of the vehicle speed control, the shift downcontrol may be utilized when a time period to the target vehicle speed,which is achieved by the full closing of the throttle, becomes greaterthan a predetermined time period. More specifically, vehicle speedcontrol block 500 may be arranged to employ the shift down control ofthe CVT in order to decelerate the vehicle at the target vehicle speedwhen the predetermined time period cannot be ensured by fully closingthe throttle.

[0102] Further, when command drive torque d_(FC)(t) is not ensured byboth the throttle control and the transmission control, and when thevehicle travels on a flat road, the shortage of command drive torqued_(FC)(t) is supplied by employing the brake system. However, when thevehicle travels on a down slop, the braking control by the brake systemis prohibited by outputting a brake control prohibiting signal BP fromcommand drive torque calculating block 530 to a command brake pressurecalculating block 630. The reason for prohibiting the braking control ofthe brake system on the down slop is as follows.

[0103] If the vehicle on the down slope is decelerated by means of thebrake system, it is necessary to continuously execute the braking. Thiscontinuous braking may cause the brake fade. Therefore, in order toprevent the brake fade, the vehicle speed control system according tothe present invention is arranged to execute the deceleration of thevehicle by means of the throttle control and the transmission controlwithout employing the brake system when the vehicle travels on a downslope.

[0104] With the thus arranged suspending method, even when the constantvehicle speed cruise control is restarted after the constant vehiclespeed cruise control is suspended in response to the temporalacceleration caused by depressing the accelerator pedal, a largerdeceleration as compared with that only by the throttle control isensured by the down shift of the transmission. Therefore, the conversiontime period to the target vehicle speed is further shortened. Further,by employing a continuously variable transmission (CVT 70) for thedeceleration, a shift shock is prevented even when the vehicle travelson the down slope. Further, since the deceleration ensured by thetransmission control and the throttle control is larger than that onlyby the throttle control and since the transmission control and thethrottle control are executed to smoothly achieve the drive torque onthe basis of the command vehicle speed variation ΔV_(COM), it ispossible to smoothly decelerate the vehicle while keeping thedeceleration degree at the predetermined value. In contrast to this, ifa normal non-CVT automatic transmission is employed, a shift shock isgenerated during the shift down, and therefore even when the largerdeceleration is requested, the conventional system employed a non-CVTtransmission has executed only the throttle control and has not executedthe shift down control of the transmission.

[0105] By employing a continuously variable transmission (CVT) with thevehicle speed control system, it becomes possible to smoothly shift downthe gear ratio of the transmission. Therefore, when the vehicle isdecelerated for continuing the vehicle speed control, a decelerationgreater than that only by the throttle control is smoothly executed.

[0106] Next, a stopping process of the vehicle speed control will bediscussed.

[0107] A drive wheel acceleration calculating block 600 of FIG. 1receives vehicle speed V_(A)(t) and calculates a drive wheelacceleration α_(OBS)(t) from the following equation (26).

α_(OBS)(t)=[K _(OBS) ·s/(T _(OBS)·s² +s+K _(OBS))]·V_(A)(t)  (26)

[0108] In this equation (26), K_(OBS) is a constant, and T_(OBS) is atime constant.

[0109] Since vehicle speed V_(A)(t) is a value calculated from therotation speed of a tire (drive wheel), the value of vehicle speedV_(A)(t) corresponds to the rotation speed of the drive wheel.Accordingly, drive wheel acceleration α_(OBS)(t) is a variation (drivewheel acceleration) of the vehicle speed obtained from the derive wheelspeed V_(A)(t).

[0110] Vehicle speed control stop deciding block 610 compares drivewheel acceleration α_(OBS)(t) calculated at drive torque calculatingblock 600 with the predetermined acceleration limit a which correspondsto the variation of the vehicle speed, such as 0.2G. When drive wheelacceleration α_(OBS)(t) becomes greater than the acceleration limit a,vehicle speed control stop deciding block 610 outputs the Vehicle speedcontrol stopping signal to command drive torque calculating block 530and target throttle opening calculating block 570. In reply to thevehicle speed control stopping signal, command drive torque calculatingblock 530 and target throttle opening calculating block 570 initializethe calculations thereof respectively. Further, when the vehicle speedcontrol is once stopped, the vehicle speed control is not started untilset switch 20 is again switched on.

[0111] Since the vehicle speed control system shown in FIG. 1 controlsthe vehicle speed at the command vehicle speed based on command vehiclespeed variation ΔV_(COM) determined at command vehicle speed variationdetermining block 590. Therefore, when the vehicle is normallycontrolled, the vehicle speed variation never becomes greater than thelimit of the command vehicle speed variation, for example,0.06G=0.021(km/h/10 ms). Accordingly, when drive wheel accelerationα_(OBS)(t) becomes greater than the predetermined acceleration limit awhich corresponds to the limit of the command vehicle speedacceleration, there is a possibility that the drive wheels are slipping.That is, by comparing drive wheel acceleration α_(OBS)(t) with thepredetermined acceleration limit α, it is possible to detect thegeneration of slippage of the vehicle. Accordingly, it becomes possibleto execute the slip decision and the stop decision of the vehicle speedcontrol, by obtaining drive wheel acceleration α_(OBS)(t) from theoutput of the normal vehicle speed sensor without providing anacceleration sensor in a slip suppressing system such as TCS (tractioncontrol system) and without detecting a difference between a rotationspeed of the drive wheel and a rotation speed of a driven wheel.Further, by increasing the command vehicle speed variation ΔV_(COM), itis possible to improve the responsibility of the system to the targetvehicle speed.

[0112] Although the embodiment according to the present invention hasbeen shown and described such that the stop decision of the vehiclespeed control is executed on the basis of the comparison between thedrive wheel acceleration α_(OBS)(t) and the predetermined value, theinvention is not limited to this and may be arranged such that the stopdecision is made when a difference between the command vehicle variationΔV_(COM) and drive wheel acceleration α_(OBS)(t) becomes greater than apredetermined value.

[0113] Command vehicle speed determining block 510 of FIG. 1 decideswhether V_(SMAX)<V_(A), that is, whether the command vehicle speedV_(COM)(t) is greater than vehicle speed V_(A)(t) and is varied to thedecelerating direction. Command vehicle speed determining block 510 setscommand vehicle speed V_(COM)(t) at vehicle speed V_(A)(t) or apredetermined vehicle speed smaller than vehicle speed V_(A)(t), such asat a value obtained by subtracting 5 km/h from vehicle speed V_(A)(t),and sets the initial values of integrators C₂(s) and C₁(s) at vehiclespeed V_(A)(t) so as to set the output of the equationC₂(s)·V_(A)(t)−C₁(s)·d_(FC)(t)=d_(V)(t) at zero. As a result of thissettings, the outputs of C₁(s) and C₂(s) become V_(A)(t) and thereforethe estimated disturbance d_(V)(t) becomes zero. Further, this controlis executed when the variation ΔV_(COM) which is a changing rate ofcommand vehicle speed V_(COM) is greater in the deceleration directionthan the predetermined deceleration, such as 0.06G. With thisarrangement, it becomes possible to facilitate unnecessaryinitialization of the command vehicle speed (V_(A)(t)→V_(COM)(t)) andinitialization of the integrators, and to decrease the shock due to thedeceleration.

[0114] Further, when the command vehicle speed (command control value ateach time until the actual vehicle speed reaches the target vehiclespeed) is greater than the actual vehicle speed and when the timevariation (change rate) of the command vehicle speed is turned to thedecelerating direction, by changing the command vehicle speed to theactual vehicle speed or the predetermined speed smaller than the actualvehicle speed, the actual vehicle speed is quickly converged into thetarget vehicle speed. Furthermore, it is possible to keep the continuingperformance of the control by initializing the calculation of commanddrive torque calculating block 530 from employing the actual vehiclespeed or a speed smaller than the actual vehicle speed.

[0115] Further, if the vehicle speed control system is arranged toexecute a control for bringing an actual inter-vehicle distance closerto a target inter-vehicle distance so as to execute a vehicle travelingwhile keeping a target inter-vehicle distance set by a driver withrespect to a preceding vehicle, the vehicle speed control system isarranged to set the command vehicle speed so as to keep the targetinter-vehicle distance. In this situation, when the actual inter-vehicledistance is lower than a predetermined distance and when the commandvehicle speed variation ΔV_(COM) is greater than the predetermined value(0.06G) in the deceleration direction, the change (V_(A)→V_(COM)) of thecommand vehicle speed V_(COM) and the initialization of command drivetorque calculating block 530 (particularly, integrator) are executed.With this arrangement, it becomes possible to quickly converge theinter-vehicle distance to the target inter-vehicle distance.Accordingly, the excessive approach to the preceding vehicle isprevented, and the continuity of the control is maintained. Further, thedecrease of the unnecessary initialization (V_(A)(t)→V_(COM)(t) andinitialization of integrators) decreases the generation of the shiftdown shock.

[0116] The entire contents of Japanese Patent Applications No.2000-143581 filed on May 16, 2000 in Japan are incorporated herein byreference.

[0117] Although the invention has been described above by reference to acertain embodiment of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiment described above will occur to those skilled in the art, inlight of the above teaching. The scope of the invention is defined withreference to the following claims.

INDUSTRIAL APPLICABILITY

[0118] A vehicle speed control system according to the present inventionis applicable to an automotive vehicle equipped with a continuouslyvariable transmission.

1. A vehicle speed control system for a vehicle, the vehicle equippedwith an engine and an automatic transmission, the vehicle control systemcomprising: a coast switch for decreasing a set vehicle speed; and acontroller connected with said coast switch, said controller,controlling a vehicle speed at the set vehicle speed by controlling athrottle of the engine and the automatic transmission, and maintaining agear ratio of the automatic transmission at the gear ratio set at themoment before decreasing the set vehicle speed when the coast switch isbeing operated to decrease the set vehicle speed.
 2. A vehicle speedcontrol system for a vehicle, the vehicle equipped with an engine and anautomatic transmission, the vehicle control system comprising: a vehiclecruise speed setting device for setting a set vehicle speed; and acontroller connected with said vehicle cruise speed setting device, saidcontroller, controlling a vehicle speed at the set vehicle speed bycontrolling a throttle of the engine and the automatic transmission,fixing a gear ratio of the automatic transmission during a time periodwhen a decreasing operation of the set vehicle speed by means of saidvehicle cruise speed device is executed.
 3. The vehicle speed controlsystem as claimed in claim 2, wherein said controller commands theautomatic transmission to prohibit from executing a shift down when theset vehicle speed is being decreased by operating said vehicle cruisespeed setting device.
 4. The vehicle speed control system as claimed inclaim 2, wherein said controller commands the engine to start adeceleration control when the set vehicle speed is being decreased byoperating said vehicle cruise speed setting device.
 5. The vehicle speedcontrol system as claimed in claim 2, wherein said vehicle cruise speedsetting device comprises a set switch for setting the set vehicle speed,a coast switch for decreasing the set vehicle speed and an accelerateswitch for increasing the set vehicle speed, said vehicle cruise speedsetting device is manually operated by a vehicle occupant.
 6. Thevehicle speed control system as claimed in claim 2, wherein saidcontroller controls the engine rotation speed within a predeterminedlimit when the vehicle speed at the moment of restarting the vehiclespeed control is greater than or equal to the target vehicle speed.
 7. Amethod for executing a vehicle speed control, comprising: controlling avehicle speed at a set vehicle speed by controlling an engine and anautomatic transmission of the vehicle; detecting whether the set vehiclespeed is being decreased; and maintaining a gear ratio of the automatictransmission at a vehicle when said vehicle cruise speed device is beingoperated to is decrease the set vehicle speed.